This invention relates to ultrasonic measurement and more particularly to ultrasonic thickness measurement conducted from one surface of a workpiece comprised of metallurgically inconsistent material or material for which a reliable ultrasonic velocity standard is not available.
Ultrasonic devices are extensively used to measure the thickness of a workpiece as shown in U.S. Pat. No. 3,642,712 to Weighart and U.S. Pat. No. 3,918,296 to Kitada and are particularly desirable when access is limited to only one surface of the workpiece to be measured. For example, in the paper industry it is necessary to periodically measure the thickness of the cast iron dryer roll shells to insure adequate thickness because they contain steam under substantial pressure. Due to the configuration of these dryer rolls, access is generally limited to the outer surface of the rolls (otherwise a destructive penetration of the roll must be performed).
Ultrasonic thickness measuring instruments have generally utilized a velocity calibration "standard" for the particular material of the workpiece to be tested since the velocity of ultrasonic waves is dependent upon the composition of the material being tested and the wave velocity in the material must be considered a known quantity to determine thickness. Inaccuracies are thus introduced into the measurement when the velocity calibration "standard" differs from the actual velocity characteristics of the workpiece. It is also common for a workpiece to exhibit varying velocity characteristics at distinct portions of the workpiece due to inherent metallurgical inconsistencies in the workpiece material such as variations in the alloy composition or in the crystalline structure.
Cast iron provides a good example of this problem because of the metallurgical inconsistency of cast iron from one sample to another due to the variations in alloying constituents (e.g. carbon, silicon, etc.) as well as variations dependent upon the manufacturing process. A workpiece of cast iron commonly exhibits metallurgical inconsistencies from one portion of the workpiece to another so as to significantly vary the velocity of ultrasonic waves at different points.
Accordingly, either the lack of an accurate velocity calibration "standard" for the particular material or the presence of metallurgically inconsistent characteristics within the workpiece material itself will introduce unacceptable error into ultrasonic measurement instruments for many critical applications.
It is an object of this invention to provide a novel improved method and apparatus for ultrasonically measuring the thickness between opposed surfaces of a workpiece with access to only one surface.
It is also an object of the invention to provide such a method and apparatus for ultrasonic measurement which provides accurate measurement of workpieces for which no reliable calibration standard exists.
Another object is to provide such a method and apparatus for ultrasonic measurement of materials which exhibit inconsistent composition and metallurgical characteristics.
A further object is to provide an apparatus for ultrasonic measurement that provides accurate, convenient, and quick multiple measurements of a large workpiece.
A still further object of the invention is to provide an ultrasonic measuring apparatus that is economical to manufacture, durable in use, conveniently operable in the field, and refined in appearance.